| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
|
|
|
|||||||
|
|||||||||
INSTITUTE FOR CRUSTAL STUDIES DEPARTMENT OF GEOLOGICAL SCIENCES UNIVERSITY OF CALIFORNIA, SANTA BARBARA, SANTA BARBARA, CALIFORNIA 93106
U.S. GEOLOGICAL SURVEY, 345 MIDDLEFIELD ROAD, MS 77, MENLO PARK, CALIFORNIA
Abstract
Direct evidence of site distortion of P- and S-wave microearthquake source spectra at Oroville, California, is presented. The data were gathered by placing vertical and three-component seismometers at 90 m intervals in a 500 m borehole through the Cleveland Hill normal fault, on which the 1975, ML = 5.7 Oroville earthquake took place. High-pressure, hydraulic locking mechanisms were used to firmly lock the seismometer packages against the borehole wall. Digital, event-triggering GEOS recorders were used to receive the data. Some 30 seismic events, including 12 microearthquakes, were recorded during the 4 months the seismometers were deployed.
By comparing the velocity spectra of microearthquake waves at the different depths, it can be seen that scattering and attenuation in the shallow crust around the borehole dominantly affect high-frequency S waves. Above 15 Hz, the value of the apparent S-wave quality factor, Qas, for the upper 500 m of crust at this site is 9. One feature of this low Qas is a gross difference between S-wave "corner" frequencies observed at ground level and 500 m downhole. For example, the uphole and downhole S-wave corner frequencies for the Mcoda = 0.4 microearthquake of Julian Day 259 differ by a factor of 2 or more. Low quality factors and depth-dependent corner frequencies were also observed for P waves, but these data are less definitive due to lower signal-to-noise ratios.
Based on their three-component particle motions and polarizations, the direct S waves of the microearthquakes appear to be composed of two phases, which have similar amplitudes but different apparent velocities. It is possible that the velocity differences are a result of anisotropy in the underlying rock.
Footnotes
* Present address: Digital Images, Suite 2050, 1616 Glenarm Street, Denver, Colorado 80202.
This article has been cited by other articles:
|
|
 |
|
  S. Castellaro and F. Mulargia VS30 Estimates Using Constrained H/V Measurements Bulletin of the Seismological Society of America, April 1, 2009; 99(2A): 761 - 773. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
R. R. Castro, C. Condori, O. Romero, C. Jacques, and M. Suter Seismic Attenuation in Northeastern Sonora, Mexico Bulletin of the Seismological Society of America, April 1, 2008; 98(2): 722 - 732. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
M. C. Chapman, P. Talwani, and R. C. Cannon Ground-Motion Attenuation in the Atlantic Coastal Plain near Charleston, South Carolina Bulletin of the Seismological Society of America, June 1, 2003; 93(3): 998 - 1011. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
Estimation of Q for Long-Period (>2 sec) Waves in the Los Angeles Basin Bulletin of the Seismological Society of America, April 1, 2003; 93(2): 627 - 638.
|
|
|
|
|
|
Observations of Earthquake Source Parameters at 2 km Depth in the Long Valley Caldera, Eastern California Bulletin of the Seismological Society of America, April 1, 2001; 91(2): 165 - 177.
|
|
|
|
|
|
R. E. Abercrombie Near-surface attenuation and site effects from comparison of surface and deep borehole recordings Bulletin of the Seismological Society of America, June 1, 1997; 87(3): 731 - 744. [Abstract] [PDF]
|
|
|
|
|
|
D. M. Boore and W. B. Joyner Site amplifications for generic rock sites Bulletin of the Seismological Society of America, April 1, 1997; 87(2): 327 - 341. [Abstract] [PDF]
|
|
|
|
|
|
M. M. Withers, R. C. Aster, C. J. Young, and E. P. Chael High-frequency analysis of seismic background noise as a function of wind speed and shallow depth Bulletin of the Seismological Society of America, October 1, 1996; 86(5): 1507 - 1515. [Abstract] [PDF]
|
|
|
|
|
|
D. Jongmans and P. E. Malin Microearthquake S-wave observations from 0 to 1 km in the Varian well at Parkfield, California Bulletin of the Seismological Society of America, December 1, 1995; 85(6): 1805 - 1820. [Abstract] [PDF]
|
|
|
|
|
|
C. J. Young, E. P. Chael, D. A. Zagar, and J. A. Carter Variations in noise and signal levels in a pair of deep boreholes near Amarillo, Texas Bulletin of the Seismological Society of America, October 1, 1994; 84(5): 1593 - 1607. [Abstract] [PDF]
|
|
|
|
|
|
J. F. Gibbs, D. M. Boore, W. B. Joyner, and T. E. Fumal The attenuation of seismic shear waves in quaternary alluvium in Santa Clara Valley, California Bulletin of the Seismological Society of America, February 1, 1994; 84(1): 76 - 90. [Abstract] [PDF]
|
|
|
|
|
|
R. J. ARCHULETA, S. H. SEALE, P. V. SANGAS, L. M. BAKER, and S. T. SWAIN Garner Valley downhole array of accelerometers: Instrumentation and preliminary data analysis Bulletin of the Seismological Society of America, August 1, 1992; 82(4): 1592 - 1621. [Abstract] [PDF]
|
|
|
|
|
|
O. BONAMASSA and J. E. VIDALE Directional site resonances observed from aftershocks of the 18 October 1989 Loma Prieta earthquake Bulletin of the Seismological Society of America, October 1, 1991; 81(5): 1945 - 1957. [Abstract] [PDF]
|
|
|
|
|
|
R. C. ASTER and P. M. SHEARER High-frequency borehole seismograms recorded in the San Jacinto Fault zone, Southern California. Part 1. Polarizations Bulletin of the Seismological Society of America, August 1, 1991; 81(4): 1057 - 1080. [Abstract] [PDF]
|
|
|
|
|
|
R. C. ASTER and P. M. SHEARER High-frequency borehole seismograms recorded in the San Jcinto Fault zone, Southern California Part 2. Attenuation and site effects Bulletin of the Seismological Society of America, August 1, 1991; 81(4): 1081 - 1100. [Abstract] [PDF]
|
|
|
|
|
|
S. BLAKESLEE and P. MALIN High-frequency site effects at two Parkfield downhole and surface stations Bulletin of the Seismological Society of America, April 1, 1991; 81(2): 332 - 345. [Abstract] [PDF]
|
|
|
|
|
|
S. H. SEALE and R. J. ARCHULETA Site amplification and attenuation of strong ground motion Bulletin of the Seismological Society of America, December 1, 1989; 79(6): 1673 - 1696. [Abstract] [PDF]
|
|
| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
